1. Field of the Invention
The present invention relates to a field emission device, and more particularly, to a field emission device having a simple structure and capable of pulse driving and local dimming.
2. Discussion of Related Art
Generally, in a field emission device, a cathode substrate having a field emitter and an anode substrate having a fluorescent layer are spaced a predetermined distance apart to face each other and vacuum-packaged, and electrons emitted from the field emitter are collided with the fluorescent layer of the anode substrate to emit light due to cathode luminescence of the fluorescent layer.
In recent times, field emission devices have received great attention as lighting devices capable of substituting for back-light units of conventional liquid crystal display (LCD) devices, surface emitting devices and lighting apparatuses.
Particularly, cold cathode fluorescent lamps (CCFLs) and light emitting diodes (LEDs) have been generally used as back-light units of the conventional LCD devices.
However, the CCFL has a complicated configuration, and thus exacts high production costs. Further, since a light source is disposed at a side of the CCFL, a large amount of power is consumed during reflection and transmission of light. Further more, use of Hg causes environmental pollution, and uniformity in brightness becomes difficult to ensure as the LCD device becomes larger.
For these reasons, recently, a field emission device having low production costs, low power consumption and relatively uniform brightness in a wide emission range has been widely used as a back-light unit of the LCD device.
A conventional field emission device will be described in detail with reference to FIG. 1.
FIG. 1 is a view of a conventional top-gate field emission device 100 having a triode structure.
Referring to FIG. 1, the conventional field emission device 100 having a triode structure includes cathode and anode substrates 110 and 130 which are spaced a predetermined distance apart to face each other, a cathode electrode 111 formed on the cathode substrate 110, a plurality of field emitters 112 spaced a predetermined distance apart from each other on the cathode electrode 111, an anode electrode 131 formed on the anode substrate 130, a fluorescent layer 132 and a metal coating layer 133 which are formed on the anode electrode 131, a gate electrode 151 interposed between the cathode substrate 110 and the anode substrate 130 to induce electron emission from the field emitter 112, a gate insulating layer 150 configured to insulate the gate electrode 151, and a spacer 160 configured to maintain a space between the gate electrode 151 and the anode electrode 131.
The metal coating layer 133 serves to reflect light emitted by colliding with the fluorescent layer 132, and a plurality of openings 150a and 151a are respectively formed in the gate insulating layer 150 and the gate electrode 151 to transmit the electron emitted from the field emitter 112.
In the field emission device 100, when a voltage difference between the cathode electrode 111 and the gate electrode 151 is equal to or higher than a threshold voltage of the field emitter 112, an electron is emitted from the field emitter 112, accelerated due to several to several tens of kV of high voltage applied to the anode electrode 131, and then collides with the fluorescent layer 132, thereby emitting light.
When such a field emission device 100 is used as a back-light unit of the LCD device, the brightness of the back-light needs to be locally controlled according to images displayed on a screen. Thus, the field emission device 100 is constructed to be capable of local dimming, which will be described below.
FIG. 2 is a view illustrating a local dimming operation of the conventional field emission device 100 of FIG. 1.
Referring to FIG. 2, the cathode electrodes 111 are disposed perpendicular to the gate electrodes 151, and then a voltage is applied to these electrodes. At this time, a cathode voltage controller 170 and a gate voltage controller 180 control the voltage to make a predetermined voltage difference between only a specific cathode electrode 111 and a specific gate electrode 151, and thus electrons are emitted from only a specific region. For example, when a driving voltage equal to or higher than a threshold voltage of the field emitter 112 is applied between an mth cathode electrode 111 and an nth gate electrode 151, only region A of the field emitter 112 emits electrons.
Here, since continuous electron emission from the field emitter 112 may degrade the field emitter 112, an electron emission amount is generally controlled by applying a pulse-type voltage to the gate electrode 151.
However, in the pulse driving method, the local dimming operation requires several to several hundreds of V of high voltage pulse to be applied to the gate electrode 151. Thus, to apply such a high voltage pulse, a pulse driving high voltage power source is separately needed, which makes a driving circuit complicated, and increases production costs.